Aqueous composition for depositing phosphors

ABSTRACT

The preparation of phosphors such as those based on zinc sulfide and/or cadmium sulfide in the form of very fine particles which include not only the sulfide but also the activator and mineralizer required to make this an active material and the deposition of such an active material from a clear solution containing the phosphor, mineralizer and dopant necessary to complete an active material, into the pores of a suitable substrate, thereby producing the essential element for a high resolution information retrieval transducer.

United States Patent Wainer 1 AQUEOUS COMPOSITION FOR DEPOSITINGPHOSPHORS [72] Inventor: Eugene Wainer, Cleveland, Ohio [73] Assignee:Horizons Incorporated, a Division of Horizons Research lncoroprated [22]Filed: Feb. 12, 1971 [21] Appl. No.: 155,662

Related U.S. Application Data [62] Division of Ser. No. 826,704, May 21,1969, Pat. No.

[52] U.S. Cl. ..252/501, 252/301.6 S, 23/134, 23/135 [51] Int. Cl ..HOlc7/08, C091: 1/12 [58] Field of Search ..2S2/50l, 301.2 R, 301.3 R,

252/30l.6 R, 301.6 S; 117/335 R, 33.5 C, 34; 23/135, 134; 106/292, 293,301

[56] References Cited UNITED STATES PATENTS 3,592,643 7/1971 Bartfai..96/1.5

2,173,895 9/1939 Booge ..106/301 X 3,127,282 3/1964 Hershinger 17/33.5 C3,347,693 10/1967 Wendland ..l l7/33.5 C 3,466,190 9/1969 Yamashita etal. ..252/501 X Primary ExaminerCharles E. Van Horn Att0meyLawrence 1.Field [5 7] ABSTRACT 4 Claims, No Drawings AQUEOUS COMPOSITION FORDEPOSITING PHOSPHORS This application is a division of application Ser.No. 826,704 filed May 21, 1969 issued Aug. 3, 1971 as U.S. Pat. No.3,597,259.

Phosphors and phosphor type materials have broad applications forvisualizing the effects of cathode rays, electron beams, x-rays,ultraviolet light, and further may be utilized in such phenomena aselectroluminescence, electroquenching, light amplifiers and lightstorage devices as has been defined previously. The class of phosphorstaken from the group of the sulfide, selenide and telluride compounds ofzinc or cadmium or combinations of zinc and cadmium, doped in variousmanners not only are used in all of the foregoing applications but whendoped in a specific manner are also useful as photoconductors.

Chemically, the base material for a large variety of presently knownphosphors is comprised of well crystallized varieties of the sulfides ofzinc or cadmium and mixtures thereof properly doped. These doping agentsare sometimes called activators and coactivators. Depending on theirchemical construction, the phosphor may act principally as the materialwhich gives off light when struck with either electromagnetic radiationor electrons or the material may act as a photo-conductor. Actuatorswhich are acceptors of electrons may consist of one or more elements orcompounds which may be chosen from the group 1-B elements or the groupV-B elements. By 1-B elements, are usually meant copper, silver, andgold. By V-B elements are usually meant phosphorus, arsenic andantimony. Similarly, a coactivator which is a donor of electrons may beadded in lieu of or in addition to the activator. The coactivator ispreferably one or more elements from the group lII-B or group VII-B,group III-B consisting of the elements of aluminum, gallium and indium,whereas group VII-B consists of the elements of chlorine, bromine andiodine. These various activators and coactivators may be provided aselements or as compounds. In some cases, a single compound, for examplecopper chloride, may furnish both an activator (copper) and acoactivator (chlorine). When the doping is carried out principally by anactivator the material tends to act as a phosphor and when the doping iscarried out principally with a coactivator the material tends to act asa photoconductor. The amount of doping or activating agent placed in thecrystal is generally in the range of 0.02 to 0.3 percent by weight ofthe host metal sulfide.

When the material is intended for use as a phosphor generally it must bewell crystallized and the activators and/or coactivators must be anintimate part of the crystal structure. When the material is intendedfor use as a photoconductor the high degree of crystallization is notusually necessary and in many cases good photoconductors are found whenthe crystallized size is of the order of 0.1 micron or very much less.The usual practice for insuring adequate crystallization of both thephosphor and the photoconducting variety of this class of compounds isto heat treat the respective purified sulfides in the presence of thevarious desired activators and/or coactivators in a fused salt bath,generally comprised of alkali halides or mixtures thereof, attemperatures in the range of 800 to 1,000 C. and to maintain thistreatment for a considerable time. The phosphor is recovered by leachingout the soluble alkali halide in water and a well crystallized rathercoarse variety of crystal is obtained.

The procedure by which phosphor particles are made in a fused saltmenstruum such as mixtures of alkali halides may be referred to as thetraditional technique. Products made by this procedure are, as indicatedpreviously, well crystallized and the particle size is quite coarse, inthe range of several microns, in order to obtain high luminescenceefficiency.'lf these coarse luminescent crystals are examined under ahigh powered microscope under conditions such that the phosphor powderis illuminated only by non-visible ultraviolet light focused on top thestage, it is seen that the source of light is such" coarse crystalsrepresents a very small portion of the total crystal indicating that thelight comes from a specificactive center very much smaller than the sizeof the host. Obviously, the efficiency of luminescence might beincreased radically if the crystal consists entirely of active centersrather than a combination of active and dead portions.

A number of techniques have been developed, therefore, to enable.phosphors to operate either as luminescent materials or aselectroluminescent materials in very thin layers. One of thesetechniques involves simultaneous evaporation of the base material plusan activator from separate boats to yield an active layer of the orderof 3 microns in thickness. This procedure is described in U.S. Pat. No.3,113,040. In order to produce a luminescent material zinc sulfide andmetallic copper are simultaneously evaporated onto a clean glasssurface. A variation of the procedure is to evaporate zinc sulfidepowder which has been previously treated to contain 0.15 percent copper,0.1 percent chlorine and0.02 percent zinc oxide. Again an active layerat a thickness of 3 microns is obtained. Highly efficientelectroluminescence is obtained when such film is placed in an electricfield, providing the evapo ration is made onto a substrate of conductiveglass. Other workers have found that when activated with a mixture ofcopper and chlorine, evaporated zinc sulfide film thicknesses of theorder of 1 micron showed high brightness efiiciency even at low voltagesand particularly in an alternating field.

Variations of this procedure have been utilized to make evaporatedlayers active for such applications as luminescence,electroluminescence, cathode luminescence, and the like as defined inU.S. Pat. No. 3,127,282. In this case the base phosphor layer is thesulfide, selenide or telluride of zinc or cadmium or combinations ofzinc and cadmium. It has been found that simple evaporation of any ofthese compounds usually produces phosphor layers of low brightnessefficiency. If, however, this thin evaporated layer is then heat treatedin an atmosphere comprising the chloride, bromide or fluoride of eitherzinc or cadmium in the presence of oxygen the efficiency of lightemission is greatly improved. In the case of zinc chloride, air ispassed over zinc chloride heated to 475 C. at reduced pressure and theatmosphere thus created contacts the phosphor layer also maintained atthis temperature. Improved results are obtained if the operation iscarried out at reduced pressure in the presence of pure oxygen.Depending on the various combinations of salts, pressure and the like,the activation to produce high efficiency light output in very thinlayers may be carried out at temperatures as low as 290 C. Thesignificance of the finding with regard to heat treatment in this halideatmosphere is that the same improvement in efficiency is obtainedwhether the base phosphor is evaporated or placed on the substrate bysettling of preformed grains.

In a variation of this procedure (US. Pat. No.

3,347,693) an exceptional increase in brightness was found by utilizingcadmium chloride not only as a flux but as a means of introducingactivator into already preformed surfaces whether these surfaces wereprepared by evaporation or from presettled grains. Normally, theluminous efficiencies 'of settled powdered surfaces are of the order of15 percent whereas the usual film deposited by evaporation has anefficiency of the order of 1 percent. Films deposited by simultaneousevaporation exhibit a brightness of approximately one-fifth thatobtained from settled powders However, the technique of simultaneousevaporation is not usually found to be too effective for zinc cadmiumsulfide phosphors which have been doped with silver, this compositionbeing the normal one used for conventional television screens. In theprocedure described in the above US. patent a layer of cadmium chlorideis first laid down on the substrate which is then covered with anevaporated layer of cadmium sulfide. An extremely thin film of silver isthen laid down on the cadmium sulfide after which the composite isheated in a nitrogen atmosphere for a few minutes at 500 C. Zinc sulfideis then evaporated over the silver film and the film composite is againthen heated in nitrogen at 500 to 550 C. It is presumed that thiscomposite method of operation, followed by the subsequent heat treatmentpermits the cadmium chloride to act as a flux for the introduction ofthe silver activator. The conversion efficiency to light through the useof electron beams with such a film is about 20 percent yielding a figureof about 5 times greater than that available from other evaporatingprocedures. For example, for a specific beam current the brightnessoutput for the normally non-flux heat treated type of evaporated surfaceis about 50 foot lamberts, whereas when treated in the manner inaccordance with the description just given the brightness output isaround 800 foot lamberts at the same beam current.

Consideration of this extremely small portion of the total prior artindicates that high brightness output efficiencies can be anticipated inextremely thin films, thin enough to be transparent, and of thicknessesof 1 micron or less, providing proper attention is placed on utilizationof not only the proper concentration of activator but means for placingthe activator in the host crystal so substantially all of the hostcrystal operates as an activated particle in contrast to the output ofcoarse crystals made by fusion treatment techniques where only a smallportion of the crystal appears to be activated.

These techniques are of particular interest relative to the problem ofproducing high brightness efficiency or other phosphor efficiencies formaterials placed in extremely small holes where the average particlesize in such holes will be substantially less than 1 micron.

However, it has been found that once the sulfides, generally along withthe desired activator and/or coactivator are made available even insubmicroscopic size and not necessarily well crystallized and with thedoping agents already incorporated in the crystal, that heat treatmentof this nonactive submicroscopic size material in an atmosphere of thehalide of one of the base constituents with or without the presence ofoxygen or other gaseous diluents, then the desired crystallization canbe made to take place at very much lower temperatures with insurancethat the doping agent is properly incorporated in the crystal. Underthese conditions crystallization can be made to take place well below600 C. and in some cases as low as 280 C., yielding materials which haveproperties comparable to those which are normally made at muchhighertemperatures. When phosphors are deposited on suitably receptivesubstrates, one of the most popular techniques presently employedinvolves the deposition of sulfides, followed by the addition of amineralizer to the sulfide and finally the addition of a doping agent.In procedures of this type it will be evident that the dopant andmineralizer are unable to completely penetrate the sulfide and are onlycapable of acting on the surface of the sulfide and possibly to a verylimited extent on those portions of the sulfide immediately below thesurface. 1

When the substrate upon which the phosphor is deposited comprisesthousands of passageways or.

channels of microscopic or submicroscopic diameter, e.g., when thesubstrate is porous, the problem is even more acute because the sulfidefills the pores and the mineralizer and dopant can reach only the top ofthe sulfide.

One aspect of the present invention relates to the preparation ofphosphors, activated and crystallized in a manner which insures that thetotal crystal is activated rather than some small portion thereof.

Another aspect of the invention resides in a technique for placingsuitably activated phosphor material in the extremely small pores ofspecially prepared substrates, which may then be constructed into usefuldevices by attachment of electrodes and other required fabricatingprocedures.

Briefly it has been found that compounds of the host materials,especially compounds of zinc and cadmium can be prepared in the form ofclear solutions by utilization of thiosulfates (dithionates) and thatthis is also true of the preferred activators such as copper, silver,gold and other noble metals. Instead of the thiosulfates, it is alsopossible to use selenosulfates or tellurosulfates instead of the morereadily available thiosulfates. Further, it has been found that thesesolutions are compatible with an excess of the halides of zinc and/orcadmium. Further, these thiosulfates or dithionate type compounds aregenerally quite soluble in weak alkali, neutral, or weak acid solutionswithout the need for complexing. When these materials do not have thedesired solubility properties in their simple form (i.e., uncomplexed)their solubility may be grossly increased by adding the comparablealkali salt as, for example, sodium thiosulfate. In many applications,the

presence of sodium salts may be considered deleterious in view of itschemical action on the host material in any subsequent heat treatingstep. To a limited extent, the sodium thiosulfate may be replaced withammonium thiosulfate but the thiosulfate of the strong base,

triphenylmethyl ammonium hydroxide is found to be a preferred substitutein place of the comparable alkali thiosulfate. Not only does the organicthiosulfate duplicate the chemical properties of the alkali sulfate butin subsequent heat treatment it is eliminated by decomposition andevaporation and does not have a deleterious effect on the host materialor on substrates such as alumina, glass or the oxides of metals, towhich the phosphor may be applied. Conversely, the stability of thesolutions and facilitation of decomposition to sulfides is also aided bythe addition of a weak acid, such as acetic, providing an ample supplyof ammonium thiosulfate is present.

The present invention utilizes the capability for placing in clearsolution not only the compounds of zinc and cadmium but along with themthe compounds of copper, silver, and a large number of other metalswhich may be utilized as activators, alongwith an excess of thechlorides of the host material so that the essential ingredients as aphosphor or photoconductor based on such materials are available as asingle dissolved solution bath. Further, these compounds may be placedin the desired anion form with the triphenylmethyl ammonium hydroxidethrough the formation of the complex indicated, thus insuring a veryactive adsorption on the active aluminum oxide surface which is producedby anodizing of aluminum metal. On heating up to temperatures usuallynot exceeding 200 C. and in many cases as low as 100 C., these saltsdecompose to their respective sulfides, selenides or tellurides. Underproper conditions the decomposition is quantitative. In the majority ofcases this decomposition takes place readily by heating in nitrogen toprevent oxidation of the respective sulfide compound. In some cases, theformation of the desired sulfide is enhanced both with respect to speedandefficiency by mixing hydrogen sulfide with the nitrogen atmosphere.

Thus, by following the teachings of this invention, not only can thesulfide be formed in dry state by thermal decomposition of thethiosulfates under mild conditions but the parent material can be placedin solution in stable manner to insure the proper placement of thesulfide in the desired area. Most importantly, however, is thecapability for including in the original soluble salt mixture the properconcentration of activator elements, thus insuring complete efficiencyof activation in the proper concentration and placement in the hostcrystal on an atomic scale. Also, preparation in this manner will permitactivation of 100 percent of the material thus eliminating the particlesize limitations of phosphors prepared by the techniques of either heattreatment and flux or even by evaporation.

Although the preferred practice of this invention is directed toplacement of such materials in a channel matrix (primarily for theelimination of crosstalk, such crosstalk representing a limitation onresolution), it is also feasible to utilize a similar procedure formaking extremely thin flat films of the material on standard substratesfor standard applications as, for example, a television screen. Thiswould eliminate the expensive and time consuming process'of evaporationand complicated heat treatment which thus far is the best approach forobtaining a high resolution output in a cathode ray tube screen.

The following examples are intended to illustrate a preferred embodimentof the present invention and are not to be construed as limiting thesame.

EXAMPLE 1 An aluminum oxide wafer is first prepared by anodizing analuminum sheet, e.g., byprocedureswell known in the artand summarized onpages 216 and 217 of the book entitled Finishing of Aluminum by S.Wernick and R. Pinner published in 1959 by Robert Draper Ltd., London.By operating according to such known procedures an aluminum oxide filmof about 50 microns thickness may be produced on the base aluminumsheet.

Thereafter the pores in the anodized layer are enlarged from initialpore sizes of about 0.03 microns to pore sizes of between 0.1 and0.4.microns, and with an open (pore) to closed (sheet) ratio between 20and 70 percent, by etching the aluminum oxide film. A preferredprocedure produces a surface with an open to closed ratio of about 50percent, pore sizes .of about 0.2 microns in a layer thickness of about50 to 60 microns. When properly prepared, this layer will withstand apotential difference across its parallel plane faces of approximately2,000 volts.

The oxide film is then stripped from the aluminum substrate on which itwas formed, by immersion in a concentrated solution of mercuricchloride, as described, for example, on page 413 of the Journal of theElectrochemical Society, Vol. 100, No. 9, September 1953.

After rinsing and drying the wafer so-prepared, the pores wereimpregnated with a clear solution containing the following:

200 ccs distilled water 60 g ammonium thiosulfate (range 30 to 100 g 15g zinc thiosulfate (range 1 to 30 g) 5 g cadmium chloride (range 0.5 to10 g) 25 g cadmium thiosulfate (range 5 to 50 g) 0.1 g silverthiosulfate and/or 0.4 g copper .thiosulfate.

While non-acid or non-alkali containing solutions can be used, usefulmodifications imparting greater stability to such forming solutions maybe imparted by adding either (1) glacial acetic acid in an amountranging between 5 ccs and 20 ccs per 200 ccs of the water content or (2)a 50 percent water solution of triphenylmethyl ammonium hydroxide in anamount ranging from 5 ccs to 50 ccs per 200 ccs of the water content.

Not only is solution stability imparted by these additions but thedecomposition to sulfides as defined in later steps is facilitated.

After immersion for about 3 minutes, the impregnated wafer was dried atabout C. for 20 minutes, permitting the salts trapped in the pores tocrystallize in the pores.

The so prepared wafer was then heat treated in an atmosphere comprisingH 8 (10 percent) and N, percent) at 200 C. for 10 minutes, after whichthe wafer was permitted to cool to room temperature. As a result of theabove treatment the pores were impregnated with a mixture of sulfides,throughout which both a dopant and mineralizer were uniformly dispersed.The particle size of the sulfides made in this manner was between 50 andangstrom units.

l060ll 0317 The wafer was then immersed in the above solution of mixedthiosulfates for a second time and the procedure of impregnating,crystallizing, heat treating, and cooling was repeated one or more timesdepending on how completely the pores are to be filled. Two cyclesresult in a pore volume filling of between about 75 to 80 volumepercent. Each additional cycle adds to the extent of filling.

At the conclusion of the last cooling step, the wafer was soaked in apercent solution of cadmium chloride in water for 2 minutes. The surfacewas wiped and permitted to dry in air. Then the wafer was given afurther heat treatment in an atmosphere consisting of H,S (10 percent)and N, (90 percent) at temperatures between 500 and 550 degrees C. forminutes.

After cooling, the wafer was ready for incorporation into an appropriateenergy conversion device.

EXAMPLE 2 The procedure of Example 1 was repeated, except that air undera reduced pressure of between 5 and 20 mm of mercury was used in thefinal heat treating step.

Once the completed phosphor has been deposited on a suitable substrateor in the pores of a porous substrate, as indicated in the precedingdescription, various kinds of high resolution devices can be preparedfrom the resulting products; usually by fabricating techniques alreadyknown in the art. Some of these devices include, for example, thefollowing:

cathode ray tube devices using high voltages or low voltages;

night vision devices; and

electroluminescent devices.

A high resolution electron beam device can be prepared by impregnatingthe pores of various wafers as described above and using the resultingproduct as the face of a cathode ray tube.

Or, metallic electrodes may be evaporated onto both planar faces of aphosphor filled wafer and the resulting product can be coated on oneface with glass and then electroded and sealed into a cathode ray tube,preferably a low voltage tube.

The phosphor filled wafers could be coated with Al or Au by evaporationand then encased in glass after suitable electrical leads had beenattached. The encapsulating glass is one which is known to transmit inthe infrared. The device is then susceptible to. activation in the nearand far infrared for use as a night illuminator.

Again the phosphor impregnated or coated. wafers prepared as describedabove could be usedin electroluminescent devices.

The invention is therefore applicable to a vast number of useful devicesand it is not intended that it be limited except as may be required bythe appended claims.

I claim:

l. A clear aqueous solution from which phosphors may be deposited ontoporous or non-porous substrates comprising:

at least one phosphor forming compound of at least one element selectedfrom the group consisting of zinc,- cadmium and mixtures of zinc andcadmium, said compound having an anion selected from the groupconsisting of thiosulfates, selenosulfates, tellurosulfates,dithionates, diselenates and ditellur tesat easi one compound of anactivator element selected from the group consisting of copper, silver,gold and the other noble metals, said compound having the same anion assaid phosphor forming compound;

at least one halide of a metal selected from the group consisting ofzinc, cadmium and mixtures of zinc and cadmium;

and including in addition a complexing agent selected from the groupconsisting of ammonium thiosulfate, ammonium selenosulfate, ammoniumtellurosulfate, the thiosulfate of triphenyl methyl ammonium hydroxide,the selenosulfate of triphenyl methyl ammonium hydroxide and thetellurosulfate of triphenyl methyl ammonium hydroxide.

2. The solution of claim 1 wherein the complexing agent is triphenylmethyl ammonium hydroxide.

3. The solution of claim 1 wherein the phosphor forming compounds andthe activator compounds are both thiosulfates.

4. The solution of claim 3 including in addition acetic acid.

2. The solution of claim 1 wherein the complexing agent is triphenylmethyl ammonium hydroxide.
 3. The solution of claim 1 wherein thephosphor forming compounds and the activator compounds are boththiosulfates.
 4. The solution of claim 3 including in addition aceticacid.